Ebola viruses are enveloped, nonsegmented, negative-strand RNA viruses, which, together with Marburg viruses, are the only two known species in the family Filoviridae (Sanchez, A. et al. 2001 in: Fields Virology, D. M. Knipe and P. M. Howley, eds. Philadelphia: Lippincott Williams & Wilkens, pp. 1279-1304). Ebola and Marburg viruses have been linked to a number of lethal outbreaks of hemorrhagic fever in humans and in non-human primates (Feldmann, H. and Klenk, H.-D. 1996 Adv Virus Res 47:1-52). Filoviruses replicate rapidly in monocytes/macrophages, and fibroblasts during the early stage of the disease (Geisbert, T. W. et al. 1992 J Comp Path 106:137-152; Feldmann, H. et al. 1996 J Virol 70:2208-2214). They also infect endothelial cells, and the accumulation of Ebola glycoproteins, coupled with the massive loss of endothelial cells, plays an important role in its pathogenicity during the latter stages of the disease (Schnittler, H. J. et al. 1993 J Clin Invest 91:1301-1309; Yang, Z.-Y. et al. 1998 Science 282:843; Yang, Z.-Y. et al. 2000 Nat Med 6:886-889). There are four well-documented Ebola virus subtypes: Ebola-Zaire, Ebola-Sudan, Ebola-Reston (Geisbert, T. W. and Jahrling, P. B. 1995 Virus Res 39: 129-150) and Ebola-Cote d'Ivoire (Leguenno, B. et al. 1995 Lancet 345:1271-1274). New outbreaks, such as those recently in Gulu, Uganda, and presently in Gabon are still emerging.
Ebola virions have a uniform diameter of 75-80 nm, with filamentous forms of approximately 970 nm in length showing peak infectivity (Geisbert, T. W. and Jahrling, P. B. 1995 Virus Res 39:129-150). An envelope putatively derived from the host cell membrane binds a 45-60 nm diameter nucleocapsid, and surface spikes are occasionally seen protruding from the viral envelope (Geisbert, T. W. and Jahrling, P. B. 1995 Virus Res 39:129-150). The 19 kb single negative-strand RNA genome encodes seven viral proteins: nucleoprotein (NP), virion protein (VP) 24, VP30, VP35, VP40, glycoprotein (GP), and RNA-dependent RNA polymerase (L) (Feldmann, H. et al. 1992 Virus Res 24:1-19; Sanchez, A. et al. 1993 Virus Res 29:215-240).
Despite the limited number of Ebola virus open reading frames, little is known about viral assembly and the control of Ebola virus replication. Some studies have suggested that replication occurs in the cytoplasm, and assembly and budding of new viral particles takes place at the plasma membrane (Feldmann, H. and Kiley, M. P. 1999 Curr Top Microbiol Immunol 235:1-21; Feldmann, H. et al. 1996 J Virol 70:2208-2214; Feldmann, H. and Klenk, H.-D. 1996 Adv Virus Res 47:1-52); however, the possibility of lytic replication and release from intracellular lysosomes cannot be excluded. Studies of Ebola and Marburg viruses using an artificial replication system based on vaccinia virus T7 expression have shown that three proteins, NP, VP35, and L, are able to support transcription of a monocistronic mini-replicon (Muhlberger, E. et al. 1998 J Virol 72:8756-8764; Muhlberger, E. et al. 1999 J Virol 73:2333-2342). More recently, an elegant reverse genetic system was utilized to generate infectious Ebola virus with a combination of viral genomic fragments and expression vectors derived from four cDNAs (NP, VP35, VP30, and L). This system was used to analyze the role of GP in viral cytopathicity in vitro (Volchkov, V. E. et al. 2001 Science 291:1965-1969).